A standard television signal, such as a signal according to NTSC standards for example, is restricted to a limited bandwidth and correspondingly exhibits limited horizontal resolution of a displayed image. The horizontal resolution of a displayed image is primarily determined by the luminance component of the television signal, which according to NTSC standards is restricted to an upper frequency limit of 4.2 MHz. Improving the resolution of an NTSC image requires that the luminance bandwidth be increased, e.g., to 6 MHz, to accommodate the extra horizontal high frequency information needed for this purpose. Such an increase in bandwidth would require that existing television standards be modified, which is an unlikely prospect.
Techniques have been proposed for compatibly encoding additional high frequency image detail information within the limits of pre-existing television signal bandwidth standards. Such encoding produces an extended definition video signal which is not only compatible with pre-existing television receivers, but also can be taken full advantage of by extended definition receivers to produce a higher resolution displayed image compared to a standard receiver. Illustratively, in one proposed encoding scheme the desired image enhancing additional high frequency information (e.g., 5-6 MHz) is selected such as by means of a bandpass filter, then heterodyned down to a lower 1 MHz frequency range within the standard 4.2 MHz video signal baseband spectrum. This 1 MHz heterodyned frequency portion then modulates a selected phase of an auxiliary subcarrier at a prescribed frequency within the standard video baseband spectrum. At a decoder in an enhanced definition television receiver, the additional high frequency information is demodulated and added to, i.e., "knitted" to, the upper portion of the high frequency band of the standard video frequency spectrum to produce a video signal with enhanced image definition. The encoded signal can otherwise be received and processed by a standard television receiver without degrading a standard displayed image, but without providing enhanced image resolution.
Encoding processes which involve a "knitting" together of signal components having different frequency spectra (after decoding) are not without problems. For example, some encoding systems require nearly ideal filters to work well. In other cases unwanted signal aliasing effects may result, such as when the signal to be encoded is heterodyned or shifted to or near DC, also due to the use of imperfect filters without a flat passband and having a very steep attenuation region. Furthermore, the main portion of the video signal spectrum to which the additional high frequency spectrum is abutted typically will have experienced significantly different signal processing, such as channel filtering for example. Such different processing is likely to produce significantly different amplitude and phase characteristics for the two signal spectrums being joined together. This is particularly true with respect to the high frequency spectrum, where the response is a function of the output filter at the transmitter and the RF (radio frequency) and IF (intermediate frequency) signal processing units at the receiver. The phase response for the high frequency spectrum is not as well controlled as that of the low frequency spectrum, and gradually worsens with increasing frequency. Also, when the main video signal and the additional high frequency component are joined together at the upper end of the high frequency spectrum of the main video signal, an amplitude dip often results around the region where the signals are joined. Even if an amplitude dip does not occur, a phase mismatch could occur. Amplitude and phase problems associated with joining or "knitting" an additional high frequency component to a main video signal are substantially eliminated by the disclosed encoding system according to the present invention, which produces an encoded television signal compatible with pre-existing television standards.